In the encoding of image sequences that represent a combination of computer image parts, for example image parts that are constructed from computer graphics, and pixel-based image parts, for example image parts that are registered by a camera, it is known to separately encode and transmit the input data of the computer graphics (script files with associated data) and the pixel-based image parts. It is only at the receiver that the graphics models are then rendered and mixed with the pixel-based image parts. Since the mixing is not a binary operation, the information about the ratio in which the mixing should be carried out must be additionally transmitted for each pixel. This information is stored in a key signal and is acquired by a color segmenting from the camera signal in traditional television (F. Fechter et al., "Signal processing for a digital HDTV chromakey mixer", Signal Processing: Image Communications 5, 1993, pp. 417-423 and T. Sprague, "Merging Digital Technology into an Analog World", SMPTE Journal, February 1994, Vol. 103, No. 2, 1994, pp. 100-104).
Methods are known for transmission that separately encode computer image parts and pixel-based image parts in a transmission unit, together with what is referred to as a key signal that indicates the ratio in which a pixel of a computer image part is to be respectively mixed with the corresponding pixel of the pixel-based image part. In blue box exposures, for example, the key signal is acquired from the pixel-based image parts by a color segmenting. The intensity of the blue coloration thereby indicates the later mixing ratio in which the pixel information of the pixel of the computer image part is mixed with the corresponding pixel information of the pixel-based image part in the receiver unit. The mixing of the two pixel information in the receiver unit is not a binary mixing.
Since the mixing is not binary, much information for the respectively mixing ratio must be transmitted for every dot with the assistance of the key signal. The key signal thus requires a considerable amount of transmission capacity. However, the information of the key signal has no additional informational content for the image itself but merely the information about the ratio in which the images should be mixed together in the receiver unit.